Tri-band filter using single stub loaded resonator

Tri-band bandpass filter using single open or short stub loaded resonators is presented in this paper. The second and the third resonant frequencies of the open stub loaded resonator can be flexibly controlled by tuning the length and impedance of the open stub, whereas the first resonant frequency is fixed. The first and the third resonant frequencies of the short stub loaded resonator can be flexibly controlled by tuning the length and impedance of the short stub, whereas the second resonant frequency is fixed. Two microstrip filters were designed and simulated, proving the concept in an excellent way. The prototype of the tri-band filters achieved insertion loss of 1.3, 1.7 and 1.5 dB at 2.4, 3.5, 5.8 GHz, 1.3, 0.8 and 1.3 dB at 1.57, 2.45, 5.2 GHz, respectively.